Due to a miniaturization and technological advancement of a mobile device, a wearable computer mountable to a living body has attracted a lot of attention. In the past, as data communications between such wearable computers, there is proposed a method in which a transceiver is connected to a computer and an electric field induced in the living body as an electric field transmittable medium by the transceiver is transmitted therein to transmit and receive data, for example, in Japanese Patent Application Laid-open Publication 2001-352298.
In an intra-body communication in which the electric field based on a signal including data to be transmitted and received is induced in the living body and communications are carried out by detecting the induced electric field, when a transceiver that is not coupled electro-statically with the earth ground is used, a favorable communication condition is realized by providing a variable reactance section between a modulation circuit and a transmitting-and-receiving electrode as shown in FIG. 1, by controlling the reactance value, and thus by increasing the electric field intensity induced in the living body.
FIG. 1 illustrates an example of a configuration of a transceiver used in the intra-body communication. Referring to FIG. 1, the transceiver is composed of an oscillator 125 outputting an alternating signal as a carrier, a modulation circuit 101 modulating the carrier using data to be transmitted, a switch 102 turning off at the time of adjusting reactance and transmitting or turning on at the time of receiving, a variable reactance section 106 causing resonance with parasitic capacitances between a living body 121 and the earth ground and also between a ground of the transceiver circuit and the earth ground, a switch 103 turning on when detecting an electric field amplitude at the time of a reactance value being large during reactance adjustment or otherwise turning off, a switch 104 turning on when detecting an electric field amplitude at the time of a reactance value being small during reactance adjustment or otherwise turning off, a filter 108 and a detector 107 detecting an electric field amplitude at the time of a reactance value being large, a filter 110 and a detector 109 detecting an electric field amplitude at the time of a reactance value being small, a differential amplifier 111 obtaining a difference between the amplitudes at the time of the reactance value being large and small, an integrator 112 integrating an output signal of the differential amplifier 111 to output a control signal for controlling reactance, a switch 105 allowing the integrator 112 to input the signal from the differential amplifier 111 during reactance adjustment and to input an electric signal from a constant voltage source 113 during transmission, the constant voltage source 113 outputting the electric signal having a voltage value of zero to the integrator 112, an adjusting signal source 114 outputting an adjusting signal for use in reactance adjustment, an adder 115 adding the adjusting signal to the control signal and outputting to the variable reactance section 106, an electric field detection optical section 116 converting an electric field induced in the living body into an electric signal, a signal processing section 117 amplifying an output signal from the electric field detection optical section 116 and performing noise elimination or the like by a filter, a demodulation circuit 118 demodulating a received signal, a waveform shaper 119 shaping a waveform, switch 120 allowing the switches 103 and 104 to input an output signal from the signal processing section 117 to the switches 103 and 104 during reactance adjustment or transmission and the demodulation circuit 118 to input the output signal during receiving, an input/output (I/O) circuit 122, a transmitting-and-receiving electrode 123, and an insulator 124.
In the transceiver having the above configuration shown in FIG. 1, a reactance value of the variable reactance is controlled so as to maximize an electric field to be induced in the living body 121. In this control, a reactance value is changed timewise from the reactance value set by a control signal. The control signal is changed to be larger when the electric field amplitude at the time of a reactance value being large is larger and to be smaller when the electric field amplitude is smaller. This operation continues to control until the amplitude becomes equal.
In FIG. 1, the electric field amplitude at the time of a reactance value being large is detected by a circuit on the switch 103 side and the electric field amplitude at the time of a reactance value being small is detected by a circuit on the switch 104 side. These values are compared by the differential amplifier 111. When the electric field amplitude at the time of a reactance value being large is larger, a positive signal is inputted to the integrator 112, thereby increasing the control signal and reducing the reactance value. When smaller, a negative signal is inputted to the integrator 112, thereby reducing the reactance value. In this method, when a magnitude relation between the adjusting signal and reactance value is correct, the reactance is automatically controlled to be the maximum.
In order to explain in detail, a waveform outputted from each component illustrated in FIG. 2A and a change of the reactance value illustrated in FIG. 2B are referred to. B1 and C1 in FIG. 2B correspond to a reactance value at the time of the adjusting signal B1 and C1, respectively. A1 is a reactance value at the beginning. In the configuration shown in FIG. 1, while the electric field amplitude is being detected, a signal is inputted to the integrator 112. When a change of the control signal is smaller than the amplitude of the adjusting signal, the reactance value at the time of C1 moves close to the reactance value at the time of A1 but is still lower than the reactance value at the time of B1. Because a relation between the adjusting signal and the reactance value is not changed, a reactance control is performed without any problem.
A waveform outputted from each component and a change in a reactance value when a change of the control signal is larger than the adjusting signal are illustrated in FIGS. 3A and 3B, respectively. B2 and C2 in FIG. 3B are a reactance value when the adjusting signals in FIG. 3A are at B2 and C2, respectively.
In addition, as the integrator 112, a signal processing circuit which has a simple circuit configuration and is suitable for circuit integration, specifically, a charge pump is often used in the past. Such a charge pump is explained in detail for example in Behzad Razavi (author), Tadahiro Kuroda (translation supervisor), “A design and application of an analog CMOS integration circuit”, Maruzen CO., LMD., March 2003, pp. 686-688.
FIG. 4 is a circuit block diagram illustrating an example of a signal processing circuit using a charge pump. The signal processing circuit 4 illustrated in FIG. 4 is comprised of two switches SW1, SW2 and a capacitor 241.
In the signal processing circuit 4, when an UP signal is inputted from outside and the switch SW1 is closed to be “on”, an electric charge flows from a voltage source Vdd having a higher voltage than a ground to the capacitor 241, thereby increasing an output voltage. Here, an “on” resistance of the switch SW1 is not zero and a timewise change of an electric charge, i.e., a current is finite. Therefore, the output voltage is not raised instantaneously to the voltage of the voltage source Vdd.
On the other hand, when a DOWN signal is inputted from outside, the switch SW2 turns on and the electric charge stored in the capacitor 241 flows to the ground, thereby reducing the output voltage.
In addition, when both switches are off (open), an amount of the electric charge stored in the capacitor 241 does not change, thereby maintaining the output voltage.
In such a signal processing circuit 4, the output voltage changes in accordance with an integration over a time period of the UP signal and the DOWN signal being inputted.
In the transceiver according to the above-stated conventional art, when a change of the control signal is larger than the adjusting signal, the reactance value at the time of the adjusting signal of C2 is larger than the reactance at the time of the adjusting signal of B2. Therefore, a magnitude relation between the adjusting signal and the reactance value is reversed, thereby jeopardizing a control to obtain the maximum value.
By the way, in order to shorten a time that is needed to maximize the amplitude of the electric field induced in the living body 121 from the beginning of reactance control, the control signal has to change largely. However, in the configuration shown in FIG. 1, the control signal cannot be changed largely, thereby prolonging a time that takes until the maximum value is obtained.
In addition, since data to be transmitted is transmitted after the reactance control is finished, if it takes a long time until the maximum value is obtained, a time to be set aside for transmitting data becomes short, thereby reducing an effective transmitting speed of data.
The above-stated signal processing circuit 4 as an integrator is often used in a Phase Locked Loop (PLL) circuit, which is an electric circuit that enables a frequency of the output signal to coincide with a reference frequency of the input signal or the like. In the PLL circuit, no large current flows from the voltage source Vdd to the ground, since the UP signal and the DOWN signal are not inputted into the circuit at the same time.
On the other hand, when the signal processing circuit 4 is applied to a circuit in which the UP signal and the DOWN signal are both inputted at the same time, the two switches SW1 and SW2 are both on. As a result, a large current can flow from the voltage source Vdd to the ground, thereby leading to a disadvantage of increased power consumption.